metamorphism: Characteristics of Metamorphism In general, a metamorphic rock is coarser and has a higher density and lower porosity than the rock from which it was formed. Under low grade metamorphic conditions, the original rocks may only compact, as in the formation of slate from shale.
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. ( This is from google so I recommend you paraphrase this )
